What ages are your children? Are you looking for books on particular birds such as robins or cardinals, or are you looking for books that deal more with birds in general dealing with such topics as feathers, flight, and different beaks?
Living books are usually written in story form by an author who is passionate about the subject. Look for books that make the subject “come alive.” Generally, you can tell if a book is living by reading a part of it. If it is interesting and you want to keep reading, it is probably a living book.
Libraries are good sources for books on nature topics. Check in the non-fiction sections for adults and children. I recommend looking in the adult section mainly because our library places books 4th grade level and up in the adult section. Also, sometimes librarians aren’t really sure of the age level of a book and sometimes a book gets placed in the wrong section.